In a communications system, intercode (or intersymbol) interference (Intersymbol Interference, ISI) is a key factor that affects system performance. To avoid the intercode interference, in the communications field, the Nyquist criterion that can avoid the ISI is proposed, and a raised cosine filter satisfying the Nyquist criterion is proposed. Currently, the Nyquist criterion for ISI-free transmission becomes a guideline for communications system design.
The Nyquist criterion can effectively avoid the ISI intercode interference and reduce detection complexity. However, in a Nyquist system, to satisfy the Nyquist criterion, excessive bandwidth inevitably needs to be introduced, thereby reducing spectral efficiency. In addition, in a current commonly used communications modulation method, there is no ISI intercode interference when each symbol is independently sent, but there is also no coding constraint relationship between transmitted symbols, thereby mitigating a capability of resisting noise and attenuation by a signal.